It is known that an automatic brake operation is established for purposes of preventing slip of the road-wheels of a vehicle when a rapid acceleration is made. An inter-vehicle distance control also utilizes such an automatic brake operation.
A known vacuum servo unit or booster is disclosed in, for example, U.S. Pat. No. 5,493,946 granted to Schluter. In this disclosed vacuum servo unit, automatic brake operation is established by introducing atmospheric pressure into a variable chamber by opening a valve. The valve is associated with a solenoid and energizing the solenoid causes atmosphere to be introduced into the variable chamber.
However, immediately upon fall opening of the valve, the vacuum servo unit outputs its maximum force. It is thus difficult in this known vacuum servo unit to establish a precise automatic braking operation based on the cause of the booster operation.
A need thus exists for a vacuum servo unit or booster that is not susceptible of the foregoing drawbacks and disadvantages.